The present invention relates to a method and device for electropneumatically testing the permeability in the longitudinal and/or transverse direction of cigarettes, including filter cigarettes and ventilated cigarettes, by subjecting each cigarette to at least one testing operation which consists of applying a pneumatic pressure to a part of the cigarette, for example to one end and/or to a portion of the wrapper, in determining the resistance or pressure drop at the input of the cigarette, transforming same into an electric measurement signal, and finally comparing said measurement signal with at least one electric reference or threshold signal.
Methods and devices of the above referred type are known, for example, from U.S. Pat. Nos. 3,690,149 and 4,227,397 and from my pending U.S. application Ser. No. 228,882 filed on Jan. 22, 1981.
It is known that in the methods and devices of known type, the value of the electric measurement signal corresponding to the pneumatic pressure which is measured, increases progressively in time until it reaches a maximum value which is substantially constant.
It is known, moreover, that upon increase of the speed of production of the cigarette-making machine which supplies the cigarettes to be tested, there is a corresponding increase of the speed of passage of the cigarettes through the device for electropneumatically testing their permeability. Consequently, the time allowed for each testing operation of each single cigarette is reduced, and also the value of the pneumatic pressure and of the corresponding electric measurement signal is reduced accordingly. As a consequence it happens that, if the electric reference or threshold signal presents a constant value, as in the known type devices, a cigarette which is considered as acceptable at a certain speed of production of the cigarette-making machine, can be rejected as not acceptable at a higher speed, or a cigarette which is considered not acceptable at a certain speed of the cigarette-making machine can become acceptable at a lower speed.
In order to obviate the above inconvenience, it has been proposed to modify the level of the electric reference signal, or the amplification (gain) of the amplifier for the measurement signals, as a function of the actual feeding speed of the cigarettes to be tested. These remedies, although successful, present the inconvenience of a construction which is rather complicated and costly, such as for example the need to utilize non-linear amplifiers, the design and the calibration of which are rather difficult whenever it is required to guarantee a constant measurement at all speeds.
The present invention has for its object to eliminate the above mentioned inconveniences and to provide, with the aid of simple components which can be easily calibrated, very precise testing of the permeability of cigarettes, at all speeds between zero speed and the maximum speed production and of feeding of the cigarettes.
According to the invention, the electric measurement signals are compared with electric reference or threshold signals which are shaped as saw-tooth signals, and are generated in synchronism with the tests of the cigarettes and consequently in synchronism with the electric measurement signals, the ascending portions of said reference signals having a rate of increase which corresponds approximately to the rate of increase of the electric measurement signals. That is, the increasing portions of the reference and measuring signals have the same approximate shape. In this case, therefore, the evaluation of the quality of the cigarette based upon the comparison between the measurement signal and the simultaneous saw-tooth reference signal is absolutely independent of the speed feeding of the cigarettes to be tested and their passage through the testing device, since both the measurement signal and the reference signal increase at least approximately at the same rate during the testing time of each cigarette.
The saw-tooth reference signals can be generated and synchronized with the measurement signals by means of any suitable devices. The invention however provides for a preferred device, in which the saw-tooth reference signals are generated with the aid of a condenser controlled by a charge switch and a discharge switch, said switches being operated by electrical pulses generated in synchronism with the rate of passage of the single cigarettes through the testing device.